A wireless communication receiver uses an analog filter to select the signal of the intended channel by removing unnecessary noise from the signal demodulated to baseband by a mixer. The accurate cutoff frequency configuration for the analog filter exerts significant influences to the system performance.
Typically, a filter has an input-to-output gain varying, as the frequency increases, and is provide with a pass band and a stop band. The term ‘cutoff frequency (fc)’ denotes a boundary frequency between the pass band and the stop band. In the case of Low Pass Filter (LPF), the cutoff frequency (fc) is defined as a frequency having a gain which is 3 dB lower than then gain of the direct current or low frequency in the pass band. The cutoff frequency (fc) is determined by a feedback resister and a feedback capacitor used in the analog filter.
The baseband covers a very broad range including 100 kHz bandwidth for the 2nd Generation (2G) communication system and 20 MHz bandwidth for the 3rd Generation (3G) and 4th Generation (4G) communication system, and the broadest bandwidth is about 100 times the narrowest one. A multimode mobile terminal designed to operate in a 2G mode for voice communication and in a 3G or 4G mode (hereinafter, referred to as 3G/4G mode) has to have a multimode multiband radio transceiver equipped with an analog baseband filter capable of supporting various bandwidths as aforementioned.
However, since the resistance and capacitance values determining the cutoff frequency of the analog baseband filter vary depending on the temperature and process conditions and are difficult to estimate accurately, the cutoff frequency is likely to differ from the target value in the real environment. Accordingly, the cutoff frequency is compensated by controlling a variable resister or a variable capacitor using a digital algorithm under the condition that the error has to be within the range of 4%.
Since the cutoff frequency is inversely proportional to the resistance and the capacitance, there is a need of the resistor having a large resistance and the capacitor having a large capacitance to process the low band signal of the legacy system such as 2G system. The capacitor for processing the low band signal of the 2G system is a few times larger in size than that for processing the 3/4G band signal and thus increases the circuit area of the analog filter. This means that the circuit area of the analog filter increases due to the disabled 2G mode in the state that the 3/4G mode is enabled, resulting in increase of manufacturing costs. The increased circuit area also elongates the wire length so as to increase signal error and noise, resulting in degradation of signal characteristics.